The conductivity of a liquid is an important analysis parameter of electrochemistry. Its measurement has wide application in fields like the chemical industry, metallurgy, biology, medicine, grain testing, water conservancy, energy resources, etc. Conductivity measuring methods can be divided into 2 groups: contact-type and non-contact type.
A non-contact type measurement applies the principle of electromagnetic induction and is therefore also referred to as an electromagnetic conductivity measuring method or an inductive conductivity measuring method. As there is no contact between the conductive part of the measuring component and the measured liquid, sensors of this type possess the advantages of good solidity, corrosion resistance, non-polarization and long service life. There has been a long history of development since the basic principle of electromagnetic measurement of the conductivity of a liquid was invented and applied in practice.
For example, U.S. Pat. No. 2,542,057 to M. J. Relis opened the basic theory to the public in 1951. The related sensor according to this reference employs a pair of coaxial toroidal cores which are covered by corrosion-protective and electrically insulating material. The inner hole of the 2 toroidal cores allows the current path through the liquid. According to the electromagnetic induction principle, when an alternating current is sent through the excitation coil, an alternating magnetic flux is generated in the excitation toroidal core, which in turn generates an induction current through the loop in the measured liquid. The induction current generated in the loop presents itself as a current loop which crosses both the excitation toroidal core and the pick-up toroidal core. This current loop generates an alternating magnetic flux in the toroidal core, which generates in the induction coil an induced current, which in turn produces an induced electrical voltage at the induction coil.
Because the induction current of the liquid is related to its conductivity, the induced current of the induction coil and the induced voltage (open-circuit voltage) is proportional to the current through the liquid. Thus, the conductivity of the liquid can be derived from the measurement of the induced current or the induced voltage. The conductivity G of the liquid is calculated from the formula G=C/R, wherein C is the sensor cell constant and R is the equivalent resistance of the loop through the liquid. As an alternative, the conductivity of the liquid can be computed from the current of the induction coil when the induced voltage at the terminal of the induction coil is zero as in the method introduced in U.S. Pat. No. 5,455,513 A1 to Neil L. Brown et al.
In the measurement of conductivity, it has also become more and more important to improve the reliability of the measuring process. If not investigated carefully, an open-circuit failure of the coils or the cable wiring could easily be mistaken for a conductivity of zero (or a very low conductivity), and thus an incorrect value may be inadvertently measured for the conductivity of the liquid.
U.S. Pat. No. 6,414,493 B1 to Behzad Rezvani discloses a method wherein an additional wire loop is introduced into the measurement sensor to form a conductive loop through the excitation coil and the induction coil. The excitation coil induces an alternating current into this wire loop, which in turn induces an additional alternating current in the induction coil to provide a base output signal. If an open circuit condition occurs in the coils, a corresponding change in the base output signal occurs, which can be detected. However, adding an additional loop to the toroidal cores makes the device more complicated.